This invention relates to the application of a carbonaceous lmaterial to a surface.
Several methods are known of applying carbon to a surface, but they may have disadvantages. If carbon is evaporated in vacuo and deposited on an electrical conductor or insulator, the deposition rate may be low and the carbon coating may not adhere firmly. If carbon from a carbon target is sputtered in an argon glow discharge on to a surface, only a small quantity of carbon is released per incident positive ion so that only a slow growth rate can be achieved, perhaps of the order of a monolayer per second. A hydrocarbon gas may be exposed to energetic electrons from a hot electron emitter or a cold cathode or radio frequency glow discharge and a carbonaceous film deposited, but when the polymer-like film which forms contains a high proportion of carbon, adhesion to the substrate may be poor. In yet another method, a metal cathode in a d.c. glow discharge may be exposed to a hydrocarbon gas so that carbon grows on the cathode, but the deposited carbon is of low electrical conductivity, which limits the conduction current and allows an accumulation of positive charge on the coated surface.
It is also known to employ a radio frequency (r.f.) system in a method of applying a carbon-containing material to a surface. In the Japanese Journal of Applied Physics, Volume 12, 1973 No. 5, page 69, Tekeda and Saito use an r.f. discharge excited by an induction coil to form polystyrene film from styrene monomer. In Thin Solid Films, Volume 23, 1974 page S45, Ando and Aozasa use a triode system with one electrode coupled to a r.f. system; a glow discharge is developed in a gas such as styrene and a film of a polymer, such as polystyrene, is deposited on the r.f. coupled electrode. However, a frequency of 100 kHz is used and it is believed that only material containing a high proportion of carbon-hydrogen bonds can be deposited, that is, a conventional high polymeric material.
A specific method of applying a layer of carbon to a substrate surface is described in U.S. Pat. No. 3,961,103, Aisenberg. A carbon-containing plasma is generated in a higher pressure plasma ion chamber. By means of an extraction electrode, positive ions are extracted from the plasma and transferred to a lower pressure vacuum deposition chamber through a constrictor means separating the two chambers. The extraction electrode forms and accelerates a beam of positively charged atomic particles and the beam then impinges on a substrate to be coated. The carbon may be introduced into the plasma chamber in the form of a hydrocarbon gas such as methane. A d.c. bias voltage may be applied to the substrate, and a radio frequency voltage may be superimposed on the bias voltage to alternately bias the substrate surface positive and negative.
In such a method, it is essential to provide two separate chambers at different pressures, to generate a plasma in one chamber, and to extract positive ions into the second chamber in a two-step process with consequent complexity of control apparatus. A further disadvantage is that, since an ion beam is used to supply the coating material, and since such beams rarely exceed 1 centimeter in diameter, only a small area of substrate can be coated. Yet another disadvantage is that during the extraction process a considerable percentage of material is lost and the positive ions in the beam form only a fraction of those available in the plasma.
It is an object of the present invention to provide a method of coating which is less complex, which is capable of operating over an area substantially greater than in the prior art, and which can operate at substantially higher deposition rates.
In this specification, the term `carbonaceous` means a material which is either pure carbon, or carbon containing a small proportion of hydrogen or other element; the number of carbon-hydrogen bonds is smaller than the number occurring in the (CH.sub.2).sub.n general structure of a high polymeric or plastics material.